Review of Grade 7: atoms, moles and classes of compounds

An atom is the smallest particle that cannot be divided in chemical changes; atoms of one kind make a chemical element, written with the first letter of its Latin name (or the first letter plus one more): H, O, Na, Ca. The real mass of an atom is tiny (about 10⁻²⁴ g), so we use the relative atomic mass Aᵣ, which shows how many times an atom is heavier than 1/12 of a ¹²C atom. The relative molecular mass Mᵣ is the sum of the Aᵣ of all atoms in the formula. One mole is the amount of substance containing 6.02·10²³ particles (the Avogadro number); the mass of 1 mole in grams is the molar mass M, numerically equal to Mᵣ (g/mol). The amount of substance is n = m : M, so m = n · M. Valency is the ability of an atom to attach other atoms: hydrogen is I, oxygen is often II.